drm/mgag200: Fix LUT programming for 16bpp

Since there are only 32 (64) distinct color values for each color
in 16bpp Matrox hardware expects those in a 'dense' manner, ie in
the first 32 (64) entries of the respective color.
Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -29,6 +29,7 @@ static void mga_crtc_load_lut(struct drm_crtc *crtc)
 	struct mga_crtc *mga_crtc = to_mga_crtc(crtc);
 	struct drm_device *dev = crtc->dev;
 	struct mga_device *mdev = dev->dev_private;
+	struct drm_framebuffer *fb = crtc->fb;
 	int i;
 
 	if (!crtc->enabled)
@@ -36,6 +37,28 @@ static void mga_crtc_load_lut(struct drm_crtc *crtc)
 
 	WREG8(DAC_INDEX + MGA1064_INDEX, 0);
 
+	if (fb && fb->bits_per_pixel == 16) {
+		int inc = (fb->depth == 15) ? 8 : 4;
+		u8 r, b;
+		for (i = 0; i < MGAG200_LUT_SIZE; i += inc) {
+			if (fb->depth == 16) {
+				if (i > (MGAG200_LUT_SIZE >> 1)) {
+					r = b = 0;
+				} else {
+					r = mga_crtc->lut_r[i << 1];
+					b = mga_crtc->lut_b[i << 1];
+				}
+			} else {
+				r = mga_crtc->lut_r[i];
+				b = mga_crtc->lut_b[i];
+			}
+			/* VGA registers */
+			WREG8(DAC_INDEX + MGA1064_COL_PAL, r);
+			WREG8(DAC_INDEX + MGA1064_COL_PAL, mga_crtc->lut_g[i]);
+			WREG8(DAC_INDEX + MGA1064_COL_PAL, b);
+		}
+		return;
+	}
 	for (i = 0; i < MGAG200_LUT_SIZE; i++) {
 		/* VGA registers */
 		WREG8(DAC_INDEX + MGA1064_COL_PAL, mga_crtc->lut_r[i]);